I enjoy both the I-16(l) and I-17. The I-16 is a fun, nimble little plane with an extremely low stall speed, and the 34mm cannon on the I-17 can one shot some planes when it connects on the right spot, but requires some level of trigger discipline.

If you go for the I-16 late, there's little noticeable change you'll need to adjust to.
The I-17 is different enough that you'll hace to learn to fly it differently. It's still nimble, but if you load the 37mm then I-16 versions will easily outturn you.